1. Field of the Invention
The present invention relates to a position measuring arrangement for detecting the relative position of a scanning unit, as well as to a reflection scale graduation, which can be moved in relation to the latter in at least one measuring direction, wherein the scanning unit includes a plurality of optical elements.
2. Discussion of Related Art
A high-resolution optical position-measuring arrangement is known from WO 02/23131 A1 and U.S. Pat. No. 7,019,842, the entire contents of which are incorporated herein by reference. Besides a reflection scale graduation embodied, for example, as a linear incident light scale, it includes a scanning unit, which can be displaced relative to it in at least one measuring direction. Besides a scanning grating and several optoelectronic detector elements, at least one optical deflection or reflector element in the form of a retro-reflector is arranged on the part of the scanning unit. A back-reflection of the partial light beams initially reflected at the reflection scale graduation in the direction of the reflection scale graduation takes place by the reflector element. There, the partial light beams are subsequently reflected a second time before interfering partial light beams then finally reach the detectors and there generate scanning signals, which are modulated as a function of their displacement. The reflector element of the position measuring arrangement in accordance with the species is designed as a ridge prism, having an optical retro-reflector functionality, and with a ridge has been aligned parallel with respect to the measuring direction. Here, the ridge prism acts as a retro-reflector in a direction which is aligned perpendicularly with respect to the measuring direction x. For creating phase-shifted scanning signals, optical polarization retardation elements in the form of small λ/4 plates are arranged in the scanning beam paths, as well as polarizers in front of the detector elements. However, such optical polarization elements basically increase the complexity, and therefore also the cost, of correspondingly constructed position measuring arrangements.
A further optical position measuring arrangement, in which a double action on the reflection scale graduation results with the aid of a retro-reflector element, is known from EP 0 387 520 A2 and U.S. Pat. No. 5,079,418, the entire contents of which are incorporated herein by reference. In connection with this scanning configuration it is required that the partial light beams impinge slanted in the line direction on the reflection scale graduation. Because of this, relatively close tolerances regarding the scanning distance result in this position measuring arrangement.
Furthermore, reference is made to DE 42 01 511 A1 and U.S. Pat. No. 5,104,225, the entire contents of which are incorporated herein by reference, in connection with such position measuring arrangements. The position measuring arrangement known from this also requires optical polarization components in the scanning beam path. It is therefore necessary to take the different diffraction efficiencies for the two polarization axes into consideration. Since the ratio of efficiency of these diffractions can greatly fluctuate over the scale graduation, it is therefore proposed in DE 42 01 511 A1 to insert two additional small λ/2 plates into the scanning beam paths. This again results in a considerable increase in the outlay, or the costs, of such systems. Furthermore, a signal improvement in this way is only possible if during both reflections the light beams are diffracted at the scale graduation with identical diffraction efficiencies for both polarization axes. However, this is not the case in actuality, since it is necessary to provide an offset of the light beams between the first reflection and the second reflection, and different diffraction conditions exist at the two reflection locations.